The present invention relates to a tunnel furnace with ultra-high frequency heating comprising sources of irradiation of microwave frequency waves which emit within a tunnel-shaped enclosure; products to be treated are caused to move through this enclosure in its longitudinal direction in order to receive the irradiated energy whose intensity is adjusted in accordance with the application contemplated (heating of products, drying, dehydration, fusion, etc.)
In the known microwave tunnel furnaces, the walls, which are generally of metal, produce the appearance of a system of interferences which make it practically impossible to obtain a constant distribution of energy in the cross sections of the enclosure; for sources disposed, as customary, in the upper part of the enclosure in planes parallel to the transverse planes, this interferential distribution with the presence of minima and maxima is accentuated by reflection of the waves on the lower wall of the tunnel. The interference fringes have positions which depend on the frequency of the waves, the position of the sources and the dimensions of the cavity of the microwave tunnel furnace. The amplitude of the undulation of these fringes is also a function of the nature of the material to be treated; the latter in fact constitutes a dampening element in the cavity of the furnace and the amount of the dampening depends essentially on the dielectric losses of the material, its water content, and its volume.
In practice it is therefore impossible to obtain a distribution of the electromagnetic field which is constant in the transverse planes of the furnace. This defect is extremely disturbing in most applications since it results in a non-homogeneous treatment of the products.